Only AMC 12 A or AMC 12 B takers who are U.S. citizens and students residing in the United States and Canada will be eligible for the USAMO.
Does that mean int’l exchange students are not allowed to participate in USAMO?
That is what it means. To my knowledge, students on an exchange program are not considered citizens.
Either way, there is another exam yet before a student is eligible for the USAMO: the American Invitational Mathematics Examination (AIME). 